Book Excerpt: The Dinosaur Fossils That Changed Everything

Footprints and Feathers on the Sands of Time

Humans have been finding the traces of extinct creatures for thousands of years. Unaware of their true identity, a variety of cultures have interpreted fossil footprints, shells and bones as the remnants of gods, heroes, saints and monsters. The cyclops, griffins and numerous other beings of myth and legend were not just figments of human imagination but monsters restored from the remains of creatures dead for millions of years. It was no different among the Native Americans of North America. The Tuscaroras, Iroquois, Onondagas and many other tribes had legends inspired by fossils, including the Lenape of the Delaware Valley.

Brian Switek
Science writer and research associate at the New Jersey State Museum, Brian Switek has done fieldwork on fossils in Utah, Montana and Wyoming. He is a frequent guest on the BBC and has written about paleontology for Smithsonian magazine, London Times, Wired Science and elsewhere. He is the author of the science blog Laelaps here on our Wired Science Blogs network and Smithsonian magazine’s Dinosaur Tracking blog.Written in Stone is his first book.

At the time Europeans arrived in North America the Lenape occupied the land from northern Delaware to the Hudson Valley of New York, and in the blood-red sandstone of this range they saw three-toed, clawed footprints. According to one tale, passed down by Richard Calmet Adams, some of these were said to be the footprints of the primeval progenitor of all the great monsters of the land and sea. It was a living horror, the destroyer of all it could dig its claws into, but perished when it was trapped in a mountain pass and obliterated by lightning.

Europeans that settled in the Connecticut Valley noticed the tracks, too. While plowing his father’s field in South Hadley, Massachusetts, around 1802 a young man named Pliny Moody turned up slabs of rock indented with weird footprints. At least one of these curiosities was appropriately put to use as a doorstep, and visitors to the Moody farm sarcastically remarked that the Pliny’s family must have raised some hearty chickens if they left footprints in solid stone. The physician Elihu Dwight, who later bought the slab, had a different interpretation. To him the tracks were made by Noah’s raven when the biblical Deluge subsided.

Such tracks were hardly unique. The abundant three-toed footprints were often called “turkey tracks” (although many indicated turkeys bigger than a full-grown human), and a cache of the impressions were discovered by laborers quarrying flagging stones near Greenfield,Massachusetts, in 1835. These were brought to the attention of the local physician James Deane, who knew they were not made by antediluvian poultry or biblical birds. Just what had created them, though, Deane could not say, and so he contacted Amherst geology professor Edward Hitchcock and Yale academic Benjamin Silliman for their opinions.

Hitchcock was initially skeptical of Deane’s claims. Some mundane geological phenomenon could have produced tracklike marks, the professor cautioned, but Deane was adamant that the footprints were genuine. Deane sent Hitchcock a cast of one of the footprints to support his case, and despite his doubts Hitchcock was intrigued. Hitchcock soon set out to have a look at the Greenfield tracks for himself and found that Deane was right. The impressions were the footsteps of ancient creatures that had trod the Connecticut Valley long before humans had settled there.

One of the many track-bearing sandstone slabs that enthralled Edward Hitchcock.

Hitchcock became enthralled by the tracks. He collected and purchased as many as he could. He fancied himself a scientific pioneer. Although Deane was also researching the tracks, Hitchcock was the first to publish on them in an 1836 issue of the American Journal of Science. There was a variety of footprint types, each given a unique binomial name to indicate a different species, but the three-toed ones were some of the most remarkable. They ranged from giant tracks over seventeen inches long to tiny impressions less than an inch from front to back. A few large slabs even showed the strides of the animals, and the only reasonable conclusion was that they had been made by birds that flocked along the ancient shoreline. “Four out of five, I presume, would draw this conclusion at once,” Hitchcock noted, and he thought that the tridactyl footprints were made by extinct equivalents of storks and herons that strode along the banks of an ancient lake or river.

Hitchcock was deeply inspired by the varied assemblage of birds that had once lived in the Connecticut Valley, and he attempted to do justice to his fascination in the anonymously published poem “The Sandstone Bird.” In the geologist’s verse, science is placed in the guise of a sorceress who conjures up the most majestic of the primeval birds:

So restored, Hitchcock’s fictional bird could only lament the dismal state of the modern world. The earth was cold and the impressive giants it knew so well were all gone. Even the trees were so Lilliputian that the dinosaur “Iguanodon could scarce here find a meal!” The haughty bird could not stand the sight of what had become of its home.

. . . all proclaims the world well nigh worn out,
Her vital warmth departing and her tribes,
Organic, all degenerate, puny soon,
In nature’s icy grave to sink forever.
Sure ’tis a place for punishment designed,
And not the beauteous happy spot I loved.
These creatures here seem discontented, sad:
They hate each other and they hate the world,
I can not, will not, live in such a spot.
I freeze, I starve, I die: with joy I sink,
To my sweet slumbers with the noble dead.

The sullen bird was then swallowed up by the earth, leaving the geologist with no evidence to prove what he had seen. Hitchcock was in a similar bind. No skeleton had been found to reveal the true form of his birds. Storks and herons provided fair analogs, but even the largest of the living wading birds was puny compared to the birds that made the largest fossil tracks. Without skeletons, Hitchcock could only guess what they looked like.

At the same time that Hitchcock was researching the Connecticut Valley tracks, Richard Owen was examining a strange chunk of bone from New Zealand. It was said to have belonged to an enormous eagle, but Owen took it to be part of the femur of a gargantuan, ostrichlike bird he called Dinornis (commonly known as the moa). From the osteological scrap he reconstructed an entire skeleton, and it was later proven to be correct when more remains of the flightless birds were found. Owen had raised a giant bird from the dead, and it provided the perfect proxy for the sandstone birds.

For Hitchcock, though, there were more than just scientific lessons to be learned from the tracks. What he saw in the fossil record spoke of God’s benevolence, and he expounded upon this belief as a Congregationalist pastor and professor of natural theology at Amherst. (Part of his inspiration for collecting so many tracks was to build a testament to God’s glorious works in nature.) He was astonished by the vast array of stupendous creatures that crawled, swam, flew, and dashed over the surface of the earth in time immemorial. Though facts from the geological strata were shaking the foundations of a literal interpretation of Genesis, Hitchcock attempted to bridge the gap between geology and theology as the Bridgewater Treatises had in England. In his Ichnology of New England Hitchcock concluded:

And how marvellous the changes which this Valley has undergone in its inhabitants! Nor was it a change without reason. We are apt to speak of these ancient races as monstrous, so unlike existing organisms as to belong to another and quite different system of life. But they were only wise and benevolent adaptations to the changing condition of our globe. One common type runs through all the present and the past systems of life, modified only to meet exigencies, and identifying the same infinitely wise and benevolent Being as the Author of all. And what an interesting evidence of his providential care of the creatures he has made, do these modifications of structure and function present! Did the same unvarying forms of organization meet us in every variety of climate and condition, we might well doubt whether the Author of Nature was also a Providential Father. But his parental care shines forth illustriously in these anomalous forms of sandstone days, and awakens the delightful confidence that in like manner he will consult and provide for the wants of individuals.

If God provided for birds that could neither sow nor reap their own food surely He would have also cared for the enormous avians of old (and even more so the human “lords of creation”). Hitchcock believed that only God could have so perfectly fitted organisms to their surroundings, but this view of nature crumbled as naturalists increasingly tried to understand nature on its own terms and not as a moral lesson. Charles Darwin’s 1859 treatise slammed the door shut on the concept of natural theology as science, which Hitchcock subscribed to, but this new perspective on life’s history raised new questions.

Birds were so different from other vertebrates that they appeared to be perched on their own lonely branch in the tree of life. How could they have evolved? Hitchcock’s tracks hinted that true birds had been present nearly as long as reptiles and amphibians, and the discovery of a fossil feather in 1860 from Solnhofen, Germany, did nothing to change this quandry. Found in the Jurassic-aged limestone of a quarry mined for stone to make lithographic plates, the delicate fossil was acquired by the German paleontologist Christian Erich Hermann von Meyer. In 1861 he named it Archaeopteryx lithographica, the “ancient feather from the lithographic limestone.”

Not long after von Meyer described the feather, another nearby limestone quarry produced an enigmatic skeleton. The jumbled creature had a long bony tail but was surrounded by feather impressions; it was as much a reptile as it was a bird. Rather than going straight to a museum, however, the specimen was given to the local physician Karl Häberlein in exchange for medical services.

Rumors of the specimen began to circulate among naturalists, but Häberlein would not part with it easily. He stipulated that the fossil would only be sold along with the rest of his fossil collection, raising the cost beyond the reach of many prospective buyers. Richard Owen and George Robert Waterhouse, certain that Archaeopteryx would bring prestige to the British Museum, were able to convince the trustees of the institution to forward £700 for the fossil (or what the museum would normally have spent on new fossil acquisitions over the course of two years). By November 1862 the fossil was in London.

Some German naturalists were upset that the slab had been expatriated to England, but the august University of Munich professor Johann Andreas Wagner had opposed efforts to acquire Archaeopteryx for his college. He was sure it was not all it seemed. Although Häberlein tried to restrict access to the specimen amid rumors it was a fake, a verbal report and sketch of the fossil reached Wagner, who argued that rather than a bird, it was a kind of reptile he called Griphosaurus, or “riddle reptile.”

Wagner’s fears over evolution had spurred his impulsive description. Archaeopteryx sounded like just the type of transitional form that would throw support to Darwin and Wallace’s evolutionary theories, and Wagner’s warnings about the fossils were among the last of his publications before his death.

Owen’s description of the fossil was read before the Royal Society in 1863. He appraised it as the “by-fossil-remains-oldest-known feathered Vertebrate.” More than that, the fossil was most certainly a bird despite its reptilian characteristics, and Owen upheld von Meyer’s original name Archaeopteryx. This diagnosis allowed Owen to make a particular prediction. The head of Archaeopteryx was missing, but Owen reasoned that “by the law of correlation we infer that the mouth was devoid of lips, and was a beak-like instrument fitted for preening the plumage of Archaeopteryx.”

While some naturalists felt that Owen’s description was rather crude, the news of the fossil was welcome among evolutionists. In an 1863 letter to Darwin the fossil mammal expert Hugh Falconer beamed,

Had the Solenhofen quarries been commissioned — by august command — to turn out a strange being à la Darwin — it could not have executed the behest more handsomely — than in the Archaeopteryx.

This news made Darwin eager to hear more about the “wondrous bird,” yet he ultimately did little to present Archaeopteryx as a confirmation of his evolutionary ideas. In the fourth edition of On the Origin of Species published in 1866, Darwin primarily used Archaeopteryx and Hitchcock’s tracks — by now thought to have been made by dinosaurs — to illustrate that the fossil record still had secrets to divulge. “Hardly any recent discovery,” Darwin wrote of Archaeopteryx, “shows more forcibly than this how little we as yet know of the former inhabitants of the world.” Even as it hinted at a connection, Archaeopteryx was too weak to unequivocally bridge the gap between reptiles and birds by itself. The necessary evidence would be supplied by the anatomist Thomas Henry Huxley.

Huxley began his scientific career in 1846 by studying marine invertebrates while serving as an assistant surgeon aboard the HMS Rattlesnake. His work was well received by other naturalists, and when he returned to England in 1850 he was set to establish himself among the scientific elite. Like the man who would become his rival, Richard Owen, Huxley was most concerned with the underpinnings of anatomical form, but where Owen cloaked his work in pious rhetoric, Huxley’s distaste for religious interference in science may have attracted him to Darwin’s theory of evolution in the first place. While Huxley disagreed with Darwin on some key points, natural selection was the best mechanism yet proposed for evolutionary change. For natural selection to make sense, however, the absence of graded transitions in the fossil record had to be accounted for, which Huxley explained through the concept of “persistent types.”

Throughout the fossil record there seemed to be little evolutionary change; crocodiles looked like crocodiles no matter what strata they came from. Instead of being evidence against evolution, however, Huxley proposed that the persistent forms were echoes of evolutionary changes that had occurred in a time so distant that it was not recorded in the rock. If most of evolution happened during “non-geologic time,” then the inability of naturalists to explain the origin of major groups of animals with fossil evidence became a moot point. The caprices of geology kept them out of science’s reach.

Thomas Henry Huxley, photographed around 1870.

This concept was a double-edged sword. It removed the problem of missing transitional forms but it made it nearly impossible to determine evolutionary relationships through fossil evidence. But Huxley was not concerned with drawing out ancestors. Instead, he was after the common denominators of animal form, and birds and reptiles provided a key example of how the same plan could be modified to different ends. During his 1863 lectures on vertebrate anatomy at the Royal College of Surgeons, Huxley asserted that birds were “so essentially similar to Reptiles in all the most essential features of their organization, that these animals may be said to be merely an extremely modified and aberrant Reptilian type.” Reptiles, too, shared similarities with birds, and to reinforce these connections Huxley placed both birds and reptiles into an encompassing group called the “Sauropsida” (thus labeling birds “reptile-faced”).

Huxley reiterated this point in his 1867 survey of birds. Reptiles and birds were modifications of the same “groundplan,” with living reptiles being closer to the hypothetical framework from which each had been adapted. If one were to compare a turtle with a dove, this association might seem laughable, but it was not among the lowly lizards and snakes that the best evidence for the connection between reptiles and birds was to be found. The solution of a fossil puzzle provided a better set of candidates.

While traveling through England that same year Huxley met geologist John Phillips, who invited Huxley to visit the museum at Oxford with him. As the naturalists strolled through the geology collection Huxley noticed something strange about the bones of the dinosaur Megalosaurus on display. A portion of its shoulder blade was actually part of the hip. Once this scrap was put in its proper place other fragments caught Huxley’s eye. When the two scientists finished reorganizing the bits of bone, they found they had restored a predator with small forelimbs and a birdlike pelvis. This new shape for Megalosaurus pointed to a deeper relationship between reptiles and birds that Huxley had inadvertently been amassing evidence for since his Royal College lectures. Dinosaurs were much more birdlike than any living reptiles, and inspired by the branching evolutionary trees in the 1866 book Generelle Morphologie by the German embryologist Ernst Haeckel, Huxley started to think of how birds actually could have evolved from reptiles. In January 1868 Huxley outlined a preliminary line of descent in a letter to Haeckel.

In scientific work the main thing just now about which I am engaged is a revision of the Dinosauria — with an eye to the Descendenz Theorie! The road from Reptiles to Birds is by way of Dinosauria to the Ratitae — the Bird “Phylum” was Struthious, and wings grew out of rudimentary fore limbs.

Huxley would unveil this evolutionary trajectory to his peers later that same year. Even though there was no direct evidence for the transition he was proposing, the forms that had already been found suggested that the connection between birds and reptiles was real. Archaeopteryx, for instance, was clearly a bird with reptilian characteristics. He conceded that it was “more remote from the boundaryline between birds and reptiles than some living Ratitae [flightless birds such as ostriches and emus] are,” and therefore not a direct ancestor of modern birds, but it still illustrated the point that birds could have evolved from reptiles. While the complete evolutionary series had yet to be found, the anatomical resemblances between flightless birds and fossil creatures like Megalosaurus suggested that the first birds had been derived from something resembling a dinosaur. This was made possible by a major change in the way paleontologists understood dinosaurs.

Two visions of Megalosaurus. While originally envisioned as an immense crocodilelike beast, as shown by the restoration on the left, by the latter part of the nineteenth century naturalists had greatly revised the appearance of the dinosaur, as shown by the restoration to the right. Unfortunately, since so little is known of Megalosaurus, we can only base our ideas of what it looked like on related theropod dinosaurs.

The first dinosaurs known to science, Megalosaurus and Iguanodon, were initially thought to have looked like enormous crocodiles and lizards. So little was known of them that they were easily cast as larger versions of known reptiles, but when Richard Owen grouped them within the Dinosauria in 1842 he gave them an anatomical overhaul. Dinosaurs, as he envisioned them, were warm-blooded creatures that carried their limbs directly beneath their bodies. They were the “highest” of the reptiles, much more impressive than their degenerate reptilian kin that inhabited the modern world, but the fragmentary nature of their remains left most of their anatomy uncertain. The discovery of a more complete dinosaur revealed that dinosaurs looked strikingly different from what Owen envisioned.

Found in the sandy marl of New Jersey in 1858, and later described by William Parker Foulke and Joseph Leidy, Hadrosaurus was a Cretaceous herbivore related to Iguanodon. Unlike Owen’s reconstruction of Iguanodon, however, its skeleton suggested that it walked upright at least some of the time. A predatory dinosaur from nearby deposits called Laelaps by its discoverer E. D. Cope (later renamed Dryptosaurus by his rival O. C. Marsh) also shattered Owen’s dinosaurian archetype. This New World relative of Megalosaurus walked on two legs, and the fact that the forelimbs of the animal were much shorter than the hind limbs caused Cope to envision an active, hot-blooded dinosaur that relied on its powerful hind limbs to kill:

This relation [between the hind limbs and forelimbs], conjoined with the massive tail, points to a semi-erect position like that of the Kangaroos, while the lightness and strength of the great femur are altogether appropriate to great powers of leaping. . . . If he were warm-blooded, as Prof. Owen supposes the Dinosauria to have been, he undoubtedly had more expression than his modern reptilian prototypes possess. He no doubt had the usual activity and vivacity that distinguishes the warm-blooded from the cold-blooded vertebrates. We can, then, with some basis of probability imagine our monster carrying his eighteen feet of length on a leap, at least thirty feet through the air, with hind feet ready to strike his prey with fatal grasp, and his enormous weight to press it to the earth. Crocodiles and Gavials must have found their bony plates and ivory no safe defence, while the Hadrosaurus himself, if not too thick skinned, as in the Rhinoceros and its allies, furnished him with food, till some Dinosaurian jackalls dragged the refuse off to their swampy dens.

Compsognathus, as restored in Huxley

If Hadrosaurus and Dryptosaurus walked on two legs it was reasonable that Iguanodon and Megalosaurus could have done the same. Three-toed tracks from the same deposits that yielded Iguanodon were in accord with the idea that it was bipedal at least some of the time, and Huxley’s own revision of Megalosaurus at Oxford suggested that it also stalked about on two legs. Yet these animals presented a substantial problem for Huxley’s evolutionary program. They were enormous animals, far too large to be good models for the forerunners of birds.

The chicken-sized dinosaur Compsognathus was a far better candidate for the sort of creatures from which birds evolved. Discovered in 1861 from the same quarries that yielded Archaeopteryx, it was more birdlike than any of its gargantuan relatives, especially in details of its hind limbs and ankles. This similarity had been recognized by the German anatomist Carl Gegenbaur in 1864, and even the anti-evolutionist Wagner drew attention to it in his description of the animal; but where Wagner disavowed that the similarities were evidence for evolution, Compsognathus was Huxley’s prime evidence that birds had sprung from reptiles. Speculating upon what it might have looked like in life, Huxley wrote:

It is impossible to look at the conformation of this strange reptile and to doubt that it hopped or walked, in an erect or semi-erect position, after the manner of a bird, to which its long neck, slight head, and small anterior limbs must have given it an extraordinary resemblance.

With this new vision of dinosaurs in place, Huxley continued to accumulate evidence that birds had been derived from the dinosaur body plan. The small dinosaur Hypsilophodon, while less birdlike than Compsognathus, was significant as it provided Huxley with the first good look at a complete dinosaurian pelvis. The process that normally extended forward in reptiles, the pubis, was rotated backward to meet the ischium, as in birds. Huxley thought it reasonable that all dinosaurs had this arrangement, and he also appealed to embryology to imply that at certain states developing chicks exhibited dinosaurlike traits.

If the whole hind quarters, from the ilium to the toes, of a half-hatched chicken could be suddenly enlarged, ossified, and fossilized as they are, they would furnish us with the last step of the transition between Birds and Reptiles; for there would be nothing in their characters to prevent us from referring them to the Dinosauria.

The English paleontologist Harry Seeley criticized this interpretation. The congruence between the hind limbs of birds and dinosaurs could be attributed to a shared mode of life, Seeley argued, and not a family relationship. In Seeley’s view, walking bipedally on land had caused the legs of both dinosaurs and birds to take similar form, and thus the resemblance was only skin-deep. This was particularly significant as Seeley had specialized in studying another group of reptiles that he thought were closer to birds.

The first pterosaur known to science was discovered in 1784 in a German limestone quarry. With a tooth-studded snout, lizardlike hind limbs, and a ludicrously long fourth finger on each hand, the creature was unlike any that had been seen before. The man who described it, Italian naturalist Cosmo Alessandro Collini, thought it was a swimmer, since it had come from marine deposits. Others disagreed and proposed that it was closely related to bats, but in 1809 Georges Cuvier recognized it as a unique kind of extinct flying reptile. He dubbed it Pterodactylus, or “wing finger.”

Not everyone was in agreement with Cuvier. In 1830, the German researcher Johannes Wagler reconstructed the animal as something of a cross between a swan and a penguin, which sculled about the surface of the water with a paddle supported by the elongated finger. Another specimen discovered in 1828 by fossil hunter Mary Anning was investigated by William Buckland. The creature was clearly a reptile, but Buckland was perplexed by its features, and he thought that, like Milton’s “Fiend” in Paradise Lost, the pterosaur could have swum, sunk, waded, crept, or flown through a strange ancient world. By the 1840s, however, there was little doubt that Cuvier had been correct, and some naturalists were very impressed by resemblances between the skeletons of the flying fiends and birds. As Richard Owen stated in an 1874 monograph of Mesozoic fossil reptiles:

Every bone in the Bird was antecedently present in the framework of the Pterodactyle; the resemblance of that portion directly subservient to flight is closer in the naked one to that in the feathered flier than it is to the forelimb of the terrestrial or aquatic reptile.

Just like Owen, Seeley saw no way to “evolve an ostrich out of an Iguanodon,” but Huxley turned the argument from convergence against his opponents. The traits supposedly shared between birds and pterosaurs had to do with flight, and given that both lineages had become adapted to flying, common traits in their skeletons were to be expected. The diagnostic traits in the hips, legs, and feet of dinosaurs, on the other hand,were found in all birds, not just ground-dwelling ones. This meant that these characters marked a true family relationship and not just a shared way of life.

To formalize this new image of dinosaurs Huxley placed them in new taxonomic groups to underline their avian characteristics. The dinosaurs and Compsognathus (which Huxley considered to be the closest relative to dinosaurs but not one itself) were put together under the name Ornithoscelida, making them the “bird-legged” members of the “reptile-faced” Sauropsida. Yet, despite all the work he had done on the topic, Huxley could not rule out any of the dinosaurs then known to be bird ancestors. Some represented the form the real ancestors may have taken, but that was all.

Huxley explained this argument in an 1870 presidential address before the Royal Society. In searching for evolutionary lineages, Huxley warned, “it is always probable that one may not hit upon the exact line of filiation, and, in dealing with fossils, may mistake uncles and nephews for fathers and sons.” To prevent this sort of confusion he drew a distinction between intercalary types, or representations of the form of ancestors and descendants, and linear types, which were the actual ancestors and descendants.

At the present moment we have, in the Ornithoscelida the intercalary type, which proves that transition [“from the type of the lizard to that of the ostrich”] to be something more than a possibility; but it is very doubtful whether any of the genera of Ornithoscelida with which we are at present acquainted are the actual linear types by which the transition from the lizard to the bird was effected. These, very probably, are still hidden from us in the older formations.

After 1870 Huxley’s paleontological work slowed.He was in over his head delivering lectures, writing papers, and engaging in the politics of science — so much so that he burned himself out. His wife, Nettie, sent him on a vacation to Egypt in 1872 with the hope that he would recover from the stress, and when Huxley returned he started on a new tack. He turned his attention to the minutiae of anatomy under the microscope, largely setting aside the old bones that had previously transfixed him.

The reconstructed skeleton of Hesperornis. As a bird with teeth, it further confirmed the connection between birds and reptiles that Huxley highlighted.

But Huxley did not abandon the evolution of birds entirely. In 1876 he set out on a lecture tour of the United States, and one of his first stops was Yale’s Peabody Museum run by American paleontologist O. C. Marsh. Though little new information about the origin of birds had been found since the time of Huxley’s 1870 address, Marsh had recently found the remains of toothed birds in the Cretaceous-age chalk of Kansas. One of the birds, Hesperornis, had tiny nubs for wings and looked like a loon with a tooth-studded beak; the other, Ichthyornis, would have looked more like a toothed gull in life.

Marsh’s odontornithes (“toothed birds”) strengthened the link between reptiles and birds, but they were geologically too young to indicate from what group birds had evolved. Along with Archaeopteryx and Compsognathus, and the early Jurassic dinosaurs which made the Connecticut Valley tracks, they could not be placed on a straight evolutionary line but instead signaled what Huxley believed was an earlier transition:

It is, in fact, quite possible that all these more or less avi-form reptiles of the Mesozoic epoch are not terms in the series of progression from birds to reptiles at all, but simply the more or less modified descendants of Palaeozoic forms through which that transition was actually effected.

We are not in a position to say that the known Ornithoscelida are intermediate in the order of their appearance on the earth between reptiles and birds. All that can be said is that if independent evidence of the actual occurrence of evolution is producible, then these intercalary forms remove every difficulty in the way of understanding what the actual steps of the process, in the case of birds, may have been.

Despite the numerous strands of evidence Huxley had tied together, the question of avian origins was far from settled, especially as his hypothetical trajectory of avian evolution came under attack. There was a growing consensus that flightless birds had evolved from flying ancestors. If this was the case, the ratites could not be used as examples of what early birds had been like. Indeed, even though the “intercalary types” identified by Huxley were important to considerations of avian evolution, there was no consensus as to how they related to each other.

The hips and hind limbs of a bird, a dinosaur (“Ornithoscelidan”), and a crocodile, as presented by Huxley in his American addresses. Huxley used this diagram to stress the birdlike nature of the hind limbs of dinosaurs.

Naturalists tried to make sense of the tangle of data in different ways. German paleontologist Carl Vogt proposed that flightless birds evolved from dinosaurs while flying birds evolved from pterosaurs. His colleague Robert Wiedersheim endorsed a modified version of this idea. Georg Baur, by contrast, thought that the backward-pointing hips of dinosaurs like Hypsilophodon and Iguanodon pinned them as ancestral to birds. One of Huxley’s pupils, E. Ray Lankester, expressed his belief that birds had evolved from aquatic dinosaurs and had wings derived from flippers.

The “Berlin” Archaeopteryx, as drawn by Gerhard Heilmann.

A second, more exquisitely preserved Archaeopteryx, discovered in 1877, fueled these continuing debates. Found in a quarry in Eichstätt, not far from Solnhofen, it is arguably the most beautiful fossil ever discovered. Whereas the “London specimen” was scrambled, the new specimen was fully articulated, its head thrown back and arms spread wide to display a splash of feathers. The fact that it had a head greatly increased its significance. Although the first specimen appeared to have been decapitated in 1865, John Evans thought that he had discovered a portion of its toothed mouth on the same slab as the rest of the skeleton. Some said that the jaws belonged to a fish, but Evans did not think it unreasonable that a bird with so many reptilian characteristics would also have teeth. The new specimen confirmed Evans’s hypothesis and refuted that of Owen. Archaeopteryx, like Hesperornis and Ichthyornis, had tooth-studded jaws. This confirmation fed into the ongoing debates of the creature’s affinities, but regardless of what group it was assigned to it was such an enigmatic fossil that it could not be ignored. In time it would be agreed that it was the very first bird, a creature that documented one point in one of life’s major transformations.

There was more to the debate than anatomy and family trees, however. The origin of birds was directly tied to questions about the origins of flight, and an early attempt to tackle this problem was undertaken in 1879 by paleontologist Samuel Williston. Taking a dinosaurian ancestry for birds as a starting point, Williston proposed:

It is not difficult to understand how the fore legs of a dinosaur might have been changed to wings. During the great extent of time in the Triassic, in which we have scanty records, there may have been a gradual lengthening of the outer fingers and greater development of the scales, thus aiding the animal in running. The further change to feathers would have been easy. The wings must first have been used in running, next in leaping and descending from heights, and, finally, in soaring.

A similar idea was later developed by the eccentric Hungarian aristocrat, spy, and paleontologist Baron Franz Nopcsa von Felsö-Szilvás. He proposed that while pterosaurs evolved from quadrupedal ancestors that lived in the trees and took to the air by leaping, birds had evolved from terrestrial predecessors that jumped and “oared along in the air” with the help of feathered arms.

Yet the “ground-up” origin for flight hypothesized by Williston and Nopcsa failed to gain a firm foothold, and other researchers continued to mull over how flight could have originated. A particularly ingenious solution to the problem was proposed by the American ornithologist William Beebe. Despite the fact that Beebe thought Archaeopteryx more of a “flutterer” than a true flyer, he believed that it might represent an early stage of flight, and he used it as a starting point to predict what its ancestors and descendants might look like.

Beebe introduced his colleagues to his hypothetical transitional series in 1915. It had all started in the trees. As Beebe had observed in the New World tropics, iguanas sometimes leapt out of trees when frightened, and when they did so they flattened themselves out to slow their descent. In such a scenario longer scales would increase their surface area to further slow their falls, Beebe reasoned, especially if these scales were situated along the arms. But the back end of the animal would have had to have been held up, too, otherwise it would be akin to a reptilian Darius Green, who, like the subject of John Townsend Trowbridge’s poem, would fall “to the ground with a thump! Flutt’rin’ an’ flound’rin’, all’n a lump!”

William Beebe’s hypothesis of the evolution of birds. According to Beebe’s scenario, bird ancestors would have started by parachuting in a “Tetrapteryx stage,” and over time the feathers of the front wings would have become enlarged, allowing for powered flight.

The key to how these hypothetical creatures stayed aloft was found in living birds. A recently hatched dove Beebe examined had rudimentary feather quills attached to its upper leg, and Archaeopteryx appeared to have long feathers on its legs, too. Thus, Beebe surmised, the ancestors of birds had leg wings that helped balance them out while parachuting and had gone through a “Tetrapteryx stage.” As the scales turned into real feathers and the animals became capable of gliding the forewings became more prominent, and the feathers of the tail became larger to support to back of the body. By combining fossil evidence with studies of living animals, Beebe was able to make a functional prediction of how birds had evolved.

Beebe’s hypothesis was only one competing among many, though, and no clear consensus could be drawn. Naturalists were unable to confirm whether flight had evolved from the “trees down” or “ground up.” Without knowledge of the ancestral form any hypothesis could be constructed from the scraps of evidence.

The skull of Euparkeria.

Older and more primitive than even the most ancient dinosaurs, the pseudosuchians seemed like good candidates for the ancestors of pterosaurs, dinosaurs, and birds. As proposed by paleontologist Robert Broom, whereas dinosaurs had peculiar specializations that would bar them from being bird ancestors, the psuedosuchians such as Euparkeria were still “generalized” creatures from which both groups could have easily derived. This would make any resemblances between birds and dinosaurs matters of convergence and not real signals of ancestry.

The Danish artist Gerhard Heilmann most forcefully articulated this hypothesis in his 1926 book The Origin of Birds. Some dinosaurs closely resembled birds, particularly the coelurosaurs such as the predatory Gorgosaurus and ostrichlike Struthiomimus, but they lacked one characteristic that barred them from a close relationship to birds: clavicles. According to Dollo’s Law, which was formulated by Belgian paleontologist Louis Dollo, evolution could not be reversed, and therefore dinosaurs could not be bird ancestors as it would require that they regrow clavicles after they had already lost them.35 This left the pseudosuchians the most appropriate stock from which to derive birds.

Heilmann’s work was a classic, and the pseudosuchian origin for birds became the favorite hypothesis during the following four decades. It was so widely accepted that even when clavicles were described among the remains of the small, predatory dinosaur Segisaurus in 1936 no one seemed to notice. (The first specimen of Oviraptor described in 1923 also had clavicles, but they were misidentified at the time.) The problem of bird origins had been solved; all that was needed were fossils to confirm the transition.

With the big question of bird ancestry seemingly resolved, work on the subject slowed during the middle of the twentieth century. Occasional alternate interpretations of Archaeopteryx continued to pop up, some closely linking the bird to dinosaurs, but the pseudosuchian hypothesis remained the favored one. Still, the resemblance between birds and predatory dinosaurs was undeniable. The immense sauropod dinosaurs were often considered to be drab, tail-dragging animals that spent much of their time in swamps, but the small predatory dinosaurs were another matter. Writing in the middle of the twentieth century, paleontologist Edwin Colbert thought the theropod Ornitholestes was an “agile” catcher of lizards and insects, and its compatriot Ornithomimus had “very long, slender hind limbs and very birdlike feet, which indicate that it must have been a rapid runner, much as are the modern ostriches.”

It would take the rediscovery of a dinosaur first found in 1931 for paleontologists to begin to fully realize the significance of the theropod dinosaurs to bird evolution. During the summer of 1964 paleontologists John Ostrom and Grant E. Meyer from Yale’s Peabody Museum were searching for fossils near the town of Bridger, Montana, when they discovered the numerous fragments of an unusual dinosaur. The famous fossil hunter Barnum Brown had found the remains of the same kind of dinosaur, which he informally called “Daptosaurus,” decades earlier, but since he never fully described it few paleontologists knew anything about it. Based upon the more complete remains they had found, however, Ostrom and Meyer knew that Brown had overlooked a dinosaur unlike any other then known.

A modern restoration of Deinonychus.

They called the new predator Deinonychus (“terrible claw”), so named because of the wicked, sickle-shaped weapon it carried on its second toe. The arrangement of the bones showed that Deinonychus held this claw off the ground, and the tail of the animal was stiffened by ossified rods that would have acted as a dynamic counterbalance. This was not a slow, stupid predator but an agile predator, and the presence of multiple individuals from the same site associated with bones of the herbivorous dinosaur Tenontosaurus suggested that Deinonychus might have been a pack hunter, something practically unheard of in dinosaurs. Of Deinonychus, Ostrom wrote:

Deinonychus must have been anything but “reptilian” in its behavior, responses and way of life. It must have been a fleet-footed, highly predaceous, extremely agile and very active animal, sensitive to many stimuli and quick in its responses. These in turn indicate an unusual level of activity for a reptile and suggest an unusually high metabolic rate.

Deinonychus stood in sharp contrast to the traditional image of dinosaurs. Even though nineteenth-century naturalists like Owen, Cope, Huxley, and Seeley thought that dinosaurs were warm-blooded animals, the consensus since that time had shifted to envision dinosaurs as larger versions of living lizards and crocodiles. Like their living counterparts they would have required a warm environment in order to be active, but the details of their physiology were unknown. What was supposed about their biology had been inferred from living reptiles in studies like that carried out on alligators by Edwin Colbert, Charles Bogert, and Raymond Cowles in 1946.

In order to approximate dinosaurian physiology, the trio of scientists carried out the unenviable task of sticking thermometers in the cloacae of American alligators. Several specimens, ranging from one to seven feet in length, were placed in the sun or shade and had their temperature taken every ten minutes. (Larger animals would have been better, but as the researchers explained, “the difficulties of making temperature experiments [on fully grown alligators] would be great and can be best left to the imagination.”) What the scientists found was that the larger alligators warmed up and cooled down slower than the smaller ones. It took about a minute and a half for the small animals to warm up one degree Celsius, while it took the largest animals five times as long to do the same. This was regulated by their internal volume. As the size of a body or object increases, its internal volume increases exponentially. An ostrich egg, for instance, is only about two and a half times as large as a chicken’s egg, yet it contains about twenty times as much fluid and tissue inside. (If you wanted to make a hard-boiled ostrich egg it would take much, much longer for the heat to cook it than it would for a chicken’s egg.) Likewise, the larger alligators had more internal volume and so took a greater amount of time to heat up or cool down. Extrapolating these differences up to the size estimates for dinosaurs, the authors wrote that it would take a ten-ton dinosaur around three and a half days of basking out in the sun to raise its body temperature one degree Celsius!

But as the researchers found out the hard way with two of their test animals, prolonged exposure to the hot sun could be deadly. It was absurd to think that dinosaurs had to sunbathe for so long to become active. (They revised their figures in later publications, writing that a large dinosaur would have to spend most of one day heating up, but this was still an unreasonable amount of time to spend sunbathing.) It was more likely that the large size of many dinosaurs shielded them from fast heat fluctuations, and that they benefited from a high, stable body temperature that would have allowed them to be active much of the time.

This only made sense for the largest dinosaurs. At only one meter tall Deinonychus was too small to maintain a near-constant high body temperature, yet it was adapted for a very active life. Was it possible that some dinosaurs maintained a high body temperature internally? Ostrom and his student Bob Bakker thought so, and French paleontologist Armand de Ricql√®s came to a similar conclusion almost simultaneously through his work on the microstructure of dinosaur bone. This launched a lively, and sometimes acrimonious, debate about the lives of dinosaurs.

After simmering for several years, the debate over “hot-blooded dinosaurs” came to a boil during a 1978 symposium hosted by the American Association for the Advancement of Science. While no clear consensus could be reached, it was apparent that the phrases “warmblooded” and “cold-blooded” were easily misused. A better understanding of the physiology of many different organisms revealed a wide diversity of metabolic strategies that were not easily categorized. An animal that controls its body temperature internally, maintains that high temperature regardless of external temperature, and has a high metabolic rate while at rest is called “endothermic.” Animals traditionally called “cold-blooded,” on the other hand, do not have constant, internally regulated body temperatures. Their metabolic rates can be high or low depending on external factors, giving them the label “ecotherms,” and they can be just as active as endothermic animals under the right conditions.

The question that remained was whether dinosaurs were endotherms or ectotherms, but without living subjects to observe it was difficult to know for sure. As paleontologist Peter Dodson opined, it was perhaps best to consider “dinosaurs as dinosaurs.” But what if dinosaurs did have living descendants, after all? The discovery of Deinonychus and the debate over dinosaur physiology reinvigorated interest in the idea that birds had evolved from dinosaurs, and if this was correct then the physiology of birds would be a model for understanding the lives of dinosaurs.

A key piece of new evidence in this reinvestigation came from a mislabeled specimen in a museum. In 1855, five years before the first Archaeopteryx feather was found, Hermann von Meyer acquired what appeared to be a pterosaur skeleton from the German limestone quarries. When Ostrom saw it over a century later, however, he knew it was no pterosaur. It was a specimen of Archaeopteryx that had been misidentified, and it was strikingly similar to Deinonychus. After carefully studying the “new” specimen, Ostrom came to the same conclusion English zoologist Percy Lowe had arrived at in 1936 (albeit by a different route). “The osteology of Archaeopteryx, in virtually every detail, is indistinguishable from that of contemporaneous and succeeding coelurosaurian dinosaurs,” Ostrom wrote, confirming that the first bird was a theropod dinosaur.

The revival of the avian dinosaur hypothesis was not immediately well received. The pseudosuchian hypothesis still held strong, even as the pseudosuchia (now sometimes called thecodontia) was recognized as a taxonomic wastebasket that did not constitute a natural evolutionary group. Slowly, however, many paleontologists came around to the view that birds might be the direct descendants of dinosaurs, even as the fossils that would confirm the transition remained elusive.

If Ostrom was right that coelurosaurs gave rise to birds, then it was likely that there were other feathered theropods waiting to be discovered. The likelihood of finding feathered dinosaurs, however, was slim. Even under the best of circumstances fossil preservation is a capricious thing. Fully articulated skeletons are rare, and rarer still are fossils that preserve any indication of body covering or soft tissues.

It was for just this reason that a snapshot circulated at the 1996 Society of Vertebrate Paleontologymeeting held at the American Museum of Natural History caught paleontologists off guard (John Ostrom among them). It showed a little theropod dinosaur not unlike Compsognathus with its head thrown back and tail pointed straight up, and along its back was a strip of fuzzy feathers. Although no scientific study had yet been undertaken (the fossil had only come to the attention of Canadian paleontologist Phil Currie and paleo-artist Michael Skrepnick two weeks earlier), the specimen confirmed the connection between dinosaurs and birds that had been proposed on bones alone. The new dinosaur was dubbed Sinosauropteryx, and it had come from Cretaceous deposits in China that exhibited a quality of preservation that exceeded that of the Solnhofen limestone.

Sinosauropteryx was only the first feathered dinosaur to be announced. A panoply of feathered fossils started to turn up in the Jurassic and Cretaceous strata of China, each just as magnificent as the one before. There were early birds that still retained clawed hands (Confuciusornis) and teeth (Sapeornis, Jibeinia), while non-flying coelurosaurs such as Caudipteryx, Sinornithosaurus, Jinfengopteryx, Dilong, and Beipiaosaurus wore an array of body coverings from wispy fuzz to full flight feathers. The fossil feathers of the strange, stubby-armed dinosaur Shuvuuia even preserved the biochemical signature of beta-keratin, a protein present in the feathers of living birds, and quill knobs on the forearm of Velociraptor reported in 2007 confirmed that the famous predator was covered in feathers, too.

A Velociraptor attempts to catch the early bird Confuciusornis. Both were feathered dinosaurs.

As new discoveries continued to accumulate it became apparent that almost every group of coelurosaurs had feathered representatives, from the weird secondarily herbivorous forms such as Beipiaosaurus to Dilong, an early relative of Tyrannosaurus. It is even possible that, during its early life, the most famous of the flesh-tearing dinosaurs may have been covered in a coat of dino-fuzz.

The coelurosaurs were among the most diverse groups of dinosaurs. The famous dinosaurs Velociraptor and Tyrannosaurus belonged to this group, as did the long-necked, pot-bellied giant herbivore Therizinosaurus and birds. What is remarkable is that, with the exception of the ornithomimosaurs, every branch on the coelurosaur family tree contains at least one feathered dinosaur, and it is expected that fossils of even more feathered coelurosaurs will be discovered as investigations continue. This suggests that, instead of evolving independently in each group, feathers were a shared trait for coelurosaurs that was inherited from their common ancestor. Most, if not all, coelurosaurs probably had some kind of feathery covering for at least part of their lives.

A mix of fossil and molecular evidence hints at how feathers could have evolved. Birds are the living descendants of the coelurosaurs, and crocodylians are the closest living relatives to dinosaurs as a whole, so features shared between birds and crocodylians might have been present in the last common ancestor of both lineages (and therefore also present in dinosaurs). Both birds and alligators, for example, share the regulatory proteins sonic hedgehog (abbreviated Shh, and named for the video game character) and bone morphogenetic protein 2 (BMP2‚Äì), both of which underlie the formation of both the scales of alligators and the feathers of birds. Hence it is likely that, during the evolution of dinosaurs, these proteins were co-opted from their roles in forming the tough hides of dinosaurs into the creation of feathers.

The diversity of feather types among coelurosaurs suggests how feathers were modified once they had begun to evolve. As seen in Sinosauropteryx, the earliest feathers were simply tubes that grew from the skin. Once these structures evolved there would have been enough variation for them to split and become branched, something that has been observed in the downy covering of baby chickens, with each feather providing greater coverage on the animal. From there, the branching filaments could be organized along a central vane, like what is seen in Caudipteryx and Sinornithosaurus. After this point, little barbs branched off from each filament along the shaft, locking them together and stiffening the feather. This was the kind of feather needed for flight, and it is what is seen in most modern birds. That these structures are feathers and not just degraded collagen or some other quirk of fossilization is beyond reasonable doubt.

The majority of dinosaur fossils are just bones and teeth, and even fossilized skin impressions only preserve patterns, not colors. But scientists have recently discovered that there is a way to detect some colors in the fossil record. While studying an exceptionally preserved fossil, squid paleontologist Jakob Vinther saw that its ink sac was packed with the same kind of microscopic spheres that give the ink of living squid their color. These bodies are called melanosomes, and once Vinther realized that they could be preserved in the fossil record he began to wonder what other prehistoric remains might contain them.

One of the first tests was on the forty-seven-million-year-old feather of an extinct bird from Messel, Germany (home of “Ida” and not far from the final resting place of Archaeopteryx). Since the feather seemed to show light and dark bands, it was a good test case to see whether the bodies were truly pigment-carrying melanosomes (in which case they would only be found in the dark bands) or were just bacterial remnants scattered all over the feather. The results were better than could have been expected. In 2009 the researchers behind the study announced that not only did the feather most certainly contain melanosomes in the dark bands, but their arrangement corresponded to a pattern seen in living birds that gives feathers a glossy sheen. This was better than just an isolated discovery. It presented paleontologists with a new technique and two teams, working independently of each other, turned to the fossilized feathers of dinosaurs to see if they, too, contained remnants of color.

The first team, lead by Fucheng Zhang, published their results in the journal Nature on January 27, 2010. They had turned their attention to two of the first feathered dinosaurs to be found, Sinosauropteryx and Sinornithosaurus. Feather samples from both contained two different types of melanosomes; those that created dark shades (eumelanosomes) and those associated with reddish hues (phaeomelanosomes). This allowed the scientists to speculate that Sinosauropteryx had a garish red-and-white-striped tail, which might have been used to signal to other members of its species.

Restorations (not to scale) of Anchiornis and Microraptor, based upon exceptional specimens that also preserved feathers. The discovery of such fossils has overwhelmingly confirmed that birds evolved from dinosaurs.

Vinther and his team published their own findings in Science the following week. Building on the previous research on the fossil bird feather, they attempted to present a specimen of the recently discovered dinosaur Anchiornis in Technicolor. After determining the pattern of melanosome distribution throughout the feathers, they compared the arrangements to what is seen in living birds to restore the long-lost pigments. As it turned out most of the feathers of Anchiornis were black, but they were set off by white accents on its wings and a plume of rufous feathers on top of its head. Even though the study did not look for chemical traces of color in the fossil that would have marked the presence of other shades, for the first time the researchers were able to produce an image of an entire living dinosaur.

The question of just what a feather is has become more complicated, however. Very early on in dinosaur evolution there was a split in the dinosaur family tree that resulted in the evolution of the ornithischians (containing an array of herbivorous dinosaurs such as the ankylosaurs, hadrosaurs, and ceratopsians) and the saurischians (comprised of the predatory theropods and the forebears of the gigantic, longnecked sauropods). The presence of feathers in coelurosaurs alone suggested that fuzzy body coverings had evolved only once among dinosaurs within the saurischian side of the split, but at the beginning of the twenty-first century scientists found similar structures among ornithischian dinosaurs. In 2002 Gerlad Mayr and colleagues announced that they had discovered a specimen of the ceratopsian Psittacosaurus with long, bristlelike structures growing out of its tail and it was joined in 2009 by Tianyulong, another bristle-covered ornithsichian described by a team of researchers led by Zheng Xiao-Ting.

A Styracosaurus, covered in bristles, scavenges the body of a dead tyrannosaur. The discovery that ornithischian dinosaurs like Tianyulong and Psittacosaurus had bristlelike structures growing out of their skin suggests that it is possible that many other ornithischian dinosaurs did, as well.

These animals were about as far removed from bird ancestry as it was possible to be while still remaining a dinosaur, yet they were covered in structures similar to the proto-feathers of Sinosauropteryx. Either the filamentous body covering evolved twice in two different groups of dinosaurs, or, even more spectacularly, was a common dinosaur trait later lost in some groups. Regardless of how many times “dino fuzz” evolved, however, these structures were only adapted into true feathers among the coelurosaurs, but how flight evolved is another evolutionary mystery.

John Ostrom presented one hypothetical scenario in 1979. Inspired by his work on Deinonychus and Archaeopteryx, he proposed that the ancestors of the first bird were small coelurosaurs covered in rudimentary feathers. With their grasping hands, these tiny predators would have been adept hunters of flying insects, and their simple feathers would have provided an unexpected advantage. The feathers along their arms would have helped trap insects, and so longer feathers would have been selected for over time. Eventually these “proto-wings” would have allowed the dinosaurs a little bit of extra lift while jumping after their prey, and this shift in selection would precipitate the origin of the first flying birds.

Ostrom’s “insect-net hypothesis” never truly took off, as it was marred by functional problems surrounding how feathers might be used as a net, but it did reignite an old debate about whether flight evolved from the “trees down” or the “ground up.” According to the advocates of the arboreal hypothesis, small feathered dinosaurs climbed up into trees and launched themselves into the air to glide a short distance, and eventually they would be adapted to beat their wings to truly fly. The four-winged dinosaur Microraptor, a relative of Deinonychus, has most recently been taken to throw support to this idea, as it may have launched itself out of trees to glide, if not truly fly, through the forest.

Other paleontologists have preferred one version or another of the cursorial hypothesis. In this view, feathered dinosaurs ran along the ground, perhaps hopping into the air after insects or other prey, until by some mechanism they developed the ability to actually fly. In fact, feathered arms may have even made some dinosaurs better runners. A key piece of evidence for this hypothesis comes from chukar partridges. These birds are capable of flight, but if they need to escape into a nearby tree or over a natural obstacle they often run rather than fly, flapping their wings as they do so. As discovered by scientist Kenneth Dial this technique gives the birds better traction while running, so much so that they can run right up vertical inclines. As hypothesized by Dial, feathered dinosaurs could have gained a functional advantage by flapping their arms while running (be it after prey or to avoid becoming prey), and this behavior could then be co-opted to allow them to start flying.

As recognized by most working paleontologists today, however, the old arboreal versus cursorial dichotomy is no longer helpful.Much like Williston, Nopsca, and Beebe, we can create numerous plausible scenarios but, without knowing which feathered dinosaurs were the root stock from which birds evolved, any origin-of-flight hypothesis must be regarded as provisional right from the start. Even as the numerous feathered fossils have confirmed that birds evolved from dinosaurs, they have also made the relationships between those fossils and birds much more complex. At one time it seemed that Velociraptor and its relatives were the closest relatives of early birds, but a little-known group of recently discovered forms may be even closer.

Described in 2002, the small feathered dinosaur Scansoriopteryx was one of the most bizarre coelurosaurs ever found. With large eyes, a short snout, and a very long third finger, this sparrow-sized dinosaur was unlike many of its coelurosaur cousins. Its description was followed in 2008 by the announcement of a close relative named Epidexipteryx, a pigeon-sized dinosaur, covered in fuzz, that also sported two pairs of ribbonlike feathers on its shortened rump and a mouth full of forwardoriented teeth. Given that they may be older than the earliest birds, they could represent the kind of dinosaur birds evolved from, in which case the Velociraptor and its relatives would be further removed from the origin of birds than had been previously supposed.

A drawing of the skeleton of Epidexipteryx, denoting the “halo” of feathers around the skeleton and the pairs of elongated feathers coming out of its tail. It may be one of the closest relatives of early birds.

For over a century Archaeopteryx was the key to understanding bird origins, as it was the oldest bird ever discovered, but as more feathered dinosaurs have been found the connection between Archaeopteryx and other fossil birds has become looser. As the delineation between non-avian dinosaur and bird has become increasingly blurred it has become difficult to tell what side Archaeopteryx falls on. As research continues, it may turn out that Archaeopteryx was, like Microraptor, a feathered dinosaur and not a true bird.

The unstable relationships of some of the feathered dinosaurs was exemplified by the redescription of Anchiornis huxleyi in 2009. The fossil, named in honor of T. H. Huxley’s work on bird origins, had been announced the year before as the closest dinosaurian relative of birds and, at thirty million years older than Archaeopteryx, was especially significant. When a better-preserved specimen was found, however, the scientists realized that their initial hypothesis was wrong. Anchiornis was actually a troodontid, or a member of a group of coelurosaurs closely related to the famous “raptors,” yet it was very similar in form to Archaeopteryx.

Even if Archaeopteryx is dethroned from the vaunted position of “earliest known bird” that Richard Owen bestowed upon it, the fact remains that birds evolved from dinosaurs, and much more than fossilized feathers supports this hypothesis. During the 1920s the explorer Roy Chapman Andrews led a string of expeditions for the American Museum of Natural History intoMongolia’s Gobi Desert to search for the evolutionary center of origin for all mammals (including humans). No evidence of a mammalian Eden was found, but the excursions did return with the ghostly white bones of the Cretaceous dinosaurs Velociraptor, Protoceratops, and Oviraptor, the latter of which was especially fascinating because it was found in the act of robbing a Protoceratops nest.

But in 1994 it was announced that the wrong dinosaur was inside the supposed Protoceratops eggs. Instead of an embryonic horned dinosaur, there was the miniscule skeleton of a developing theropod much like Oviraptor. The specimen the Andrews expedition had found was probably caring for its own eggs, not robbing the eggs of others. The discovery of several skeletons of a crested Oviraptor relative named Citipati, which were found sitting atop nests of the same kind of eggs, supported this hypothesis. Their arms encompassed the sides of the nest in a position only seen in birds, and the close relationship between Citipati and the feathered Caudipteryx opened the possibility that these dinosaurs, too, were covered in feathers that they used to regulate the temperature of their nests. This discovery of fossilized behavior dovetailed beautifully with the numerous feathered coelurosaurs, and the description of the tiny troodontid Mei long in 2004 also surprised paleontologists. Like the skeletons of Citipati on their nests, several of these dinosaurs were suddenly killed and buried while sleeping, perfectly preserved in the position in which they died. They were curled up just like slumbering birds.

The unique breathing system seen in modern birds also appeared long before their ancestors first took to the air. As you relax reading this book you go through a breathing cycle of inhaling and exhaling. When you inhale, air enters your lungs (where oxygen is absorbed), and when you exhale the carbon dioxide-rich, oxygen-depleted air is forced out. Unlike you, however, birds lack a diaphragm and cannot inflate or deflate their lungs. Instead birds have a “one way” breathing system in which fresh air moves through their respiratory system both when the bird inhales and exhales. This is made possible by a series of anterior and posterior air sacs that can expand and contract. This is a more efficient way of getting oxygen from the air, but these air sacs also have a structural benefit. They arise from the lungs and invade the surrounding bones, thus making birds lighter. This infiltration into the bone leaves telltale hollows and indentations on bones, which have been seen in dinosaurs for over one hundred and fifty years.

A reconstruction of the skeleton of Majungasaurus, showing the placement of air sacs within the body inferred from pockets in its bones. Although Majungasaurus was not closely related to birds, the presence of these structures in its skeleton shows that these features were widespread among saurischian dinosaurs.

It might not come as a surprise that coelurosaurs have evidence of air sacs on their bones, but other saurischian dinosaurs shared the same feature, too. This makes sense given the evolutionary history of these dinosaurs. There is no sign that air sacs were present in the ornithischian dinosaurs, but the evidence for air sacs in saurischian dinosaurs goes all the way back to one of the earliest presently known. Called Eoraptor, this small bipedal dinosaur was not unlike Compsognathus,and it may be a fair approximation of what some of the earliest saurischian dinosaurs were like. Its bones were marked by indentations that indicate that it had at least some rudimentary air sacs, and later predatory dinosaurs from the coelurosaurs to the knobbly-headed abelisaur Majungasaurus and the Allosaurus-relative Aerosteon had even better developed air sacs.

The other great saurischian dinosaur group, the sauropods, also had bones infiltrated by air sacs. If you tried to design an animal like a 100-foot-long sauropod with thick, heavy bones in its neck, it would have been unable to lift its head. Much like a bridge, their skeletons reflect the selective pressures for strength and lightness, and air sacs allowed them to achieve this. They probably inherited this trait from their last common ancestor with the theropod dinosaurs.

While not exactly like those seen in living birds, the air sacs in many of these saurischian dinosaurs may have also provided them physiological benefits. Air sacs may have initially been selected because they lightened the skeleton, but if they provided dinosaurs with more efficient breathing (allowing them to be more active, for example) there would have been additional benefits for natural selection to act upon. Research into this area is still new, but it is clear that rudimentary air sacs appeared in dinosaurs seventy-five million years before Archaeopteryx, long preceding the first birds.

Some dinosaurs were even plagued by parasites that now infest the mouths of living birds. From healed wounds on skulls paleontologists have known for years that large predatory dinosaurs bit each other on the face during combat. Tyrannosaurs, especially, showed scars from such conflicts, but many Tyrannosaurus jaws often had holes in the lower jaw not apparently caused by the teeth of a rival. When paleontologists EwanWolff, Steven Salisbury, Jack Horner, and David Varricchio took another look at the jaws of tyrannosaurs that had these holes they did not find any sign of infection, inflammation, or healing that would be expected if the dinosaurs had been bitten. Bone, after all, is living tissue, and would slowly remodel itself in the wake of an injury. Instead, the holes were smooth, as if the bone was being slowly eaten away.

The lower jaw of a hawk compared with the lower jaw of a Tyrannosaurus rex, both showing lesions in the bone caused by the microorganism Trichomonas gallinae.

It seemed more likely that the holes were the result of some kind of pathology, and the researchers found that the sores were consistent with damage done by a single-celled protozoan called Trichomonas gallinae that infests modern birds. When inside living birds this microscopic creature causes ulcers to form in the upper digestive tract and mouth of the host, virtually identical to the damage seen in the Tyrannosaurus jaws. The species of protozoan that afflicted Tyrannosaurus might have been only a close relative of the living kind, but this was the first evidence of an avian disease afflicting dinosaurs.

Traits we think of as clearly identifying birds — feathers, air sacs, behavior, and even peculiar parasites — were present in a wide variety of dinosaurs first. Distinguishing the first true birds from their feathered dinosaur relations has become increasingly difficult. If we define birds as warm-blooded, feathered, bipedal animals that lay eggs, then many coelurosaurs are birds, so we have to take another approach.

Living birds, from kiwis to chickadees, fall within the group Aves, which also includes extinct birds like Confuciusornis, Jeholornis, Zhongornis, Lonipteryx, Hesperornis, and Archaeopteryx. Overall, Aves is the taxonomic equivalent of what are often informally referred to as “birds,” but the earliest birds share many features with their closest relatives among the non-avian dinosaurs. What the closest dinosaurian relatives of birds may be, though, is presently under debate. Deinonychosaurs, the group containing both the dromaeosaurs (i.e. Deinonychus, Microraptor) and troodontids (Mei, Anchiornis), has often had pride of place as the dinosaurs nearest to birds and the group from which birds evolved. The identification of Archaeopteryx as a feathered dromaeosaur certainly reinforces this view, but the research describing the dinosaurs Scansoriopteryx and Epidexipteryx has placed them even closer to birds than the dromaeosaurs.

If the new analyses are supported by further evidence, Scansoriopteryx and Epidexipteryx together would make up a group called the Scansoriopterygidae and be the closest relatives to Aves. Thus, Aves plus the Scansoriopterygidae would form a group called the Avialae, with the deinonychosaurs being the next closest relatives to both groups. This placement does not reveal direct ancestors and descendants, but rather represents the group of dinosaurs from which birds arose and what they might have looked like. It is extremely unlikely that a direct line of descent from birdlike dinosaur to the first dinosaurlike bird will be found.

In his 1871 critique of evolution by natural selection, On the Genesis of Species, George Jackson Mivart considered the wings of birds a damning example of how Darwin’s theory failed. To him a bird’s wing was an atrophied organ, degenerate in the number of digits and bones in each finger. “Now, if the wing arose from a terrestrial or subaerial organ, this abortion of the bones could hardly have been serviceable — hardly have preserved individuals in the struggle for life.” In other words, how could organisms have survived with half-formed wings?

A simplified evolutionary tree of theropod dinosaurs highlighting the relationships of coelurosaurs and birds.

What we know now about evolution has undermined Mivart’s contention. The limbs of birds are only the modified limbs of dinosaurs; all the bones in the wing of a bird were present in the terrible, grasping hands of Deinonyonus and the delicate manus of Epidexipteryx. There is scarcely anything about a pigeon perched on a statue or a chicken you eat for dinner that did not first appear in dinosaurs, long before Confuciusornis flew in great flocks over what is now China. The majority of their relatives sunk into extinction sixty-five million years ago, but they are perhaps the most successful dinosaurs ever to have evolved. If you want to see living dinosaurs, you don’t have to trek to a steaming jungle or isolated plateau. All you have to do is put up a bird feeder and look out the window.

But dinosaurs and birds were not the only terrestrial vertebrates evolving during the Mesozoic. The first mammals evolved alongside early dinosaurs, but they remained small creatures that lived in the corners of the world’s ecosystems. The worst mass extinction ever to strike the planet had almost entirely wiped out their ancestors, making them only the remnants of a family that once flourished, but, 150 million years later, a stroke of bad luck for the dinosaurs would prove to be an unexpected boon.